1. Field of the Invention
The present invention relates generally to the packaging field. More particularly, it concerns monovinylarene-conjugated diene copolymers and compositions thereof.
2. Description of Related Art
Shrink labels are known for use in labeling preformed packages or sealing the closures of preformed packages. Examples of such uses of shrink labels include, but are not limited to, labels on contoured bottles of beverages and containers of prepackaged foods and tamper-evident seals over the caps of pharmaceutical bottles, among others.
Two materials frequently used in shrink labels are polyvinylchloride (PVC) and glycol-modified polyethylene terephthalate glycol (PETG). PETG is generally considered to have a relatively low glass transition temperature and a relatively high shrinkage upon exposure to heat, and as a result has been frequently used in shrink film applications. However, PETG is relatively expensive and has low melt strength, low heat deflection temperature, a tendency to absorb moisture, and relatively high specific gravity. Polyvinyl chloride (PVC) is relatively inexpensive, and has been used in shrink label packaging applications, but its poorer shrink performance, relatively high specific gravity, formation of corrosive and hazardous gases during extrusion, and a negative public perception of plasticizers often used to soften PVC and several of its incineration products render it less desirable.
Styrenic polymers, including styrene-butadiene copolymers, do not have these disadvantages. However, it remains challenging to produce shrink labels containing styrene-butadiene copolymers that have clarity, toughness, and shrink properties competitive with PVC or PETG.